A clusterable sigraph is a sigraph in which its vertex set is partitioned into pairwise disjoint subsets in
such a way that whenever there are positive edges they lie in the same subset and negative edges lie
in the different subsets. These subsets are called clusters (see [1]}). A sigraph is called
sign-compatible [2]} if it is possible to mark the vertices of the sigraph in such a manner
that for every negative edge there are negatively marked vertices on its both ends and for any
positive edge its ends are not assigned negative, sign-incompatible otherwise.
